With the rapid development of the wireless technology, the wireless network, in particular WLAN (Wireless Local Area Network) based on IEEE802.11 standard protocol has been widely used. But its service coverage area is limited due to limitation of WLAN power. When the mobile station leaves the current AP (Access Point) coverage area or loses contact with it, it must disconnect current association and re-associate with another AP, resulting in the phenomenon of handover.
IEEE802.11 standard mainly defines the specifications of physical layer and medium access control layer of wireless local area network transmission, but it does not specifies how mobile station or terminal handovers among wireless APs. However, the standard specifies the processes for synchronization, authentication, association and re-association. According to the protocol, the handover process of the wireless local area network consists of the following three phases: